A study of the genomic basis of symbiosis in a deep-sea tubeworm

Deep-sea chemosynthesis-based ecosystems are often populated by high densities of invertebrate animals. These animals host symbiotic bacteria that can provide them with energy and nutrition. The symbionts use simple chemicals such as methane, hydrogen sulfide, carbon monoxide and hydrogen released from the seabed as energy sources for chemosynthesis, allowing their invertebrate hosts to thrive in the extreme environments with very limited access to photosynthesis-derived organic matter produced in the surface water. Previous studies have sequenced the genomes of several endosymbiotic bacteria of deep-sea invertebrates such as siboglinid tubeworms, mussels and clams, providing valuable insights into the host-symbiont relationship from the perspectives of the symbionts. Nevertheless, within the four evolutionary lineages of tubeworms (Vestimentifera, Frenulata, Osedax and Sclerolinum) in the family Siboglinidae, none of the Sclerolinum species have a sequenced symbiont genome, making it difficult to test hypotheses about the evolutionary origin of the symbiosis and how siboglinids have adapted to live in various chemosynthesis-based ecosystems including hydrothermal vents, cold seeps, whale falls, wood falls and muddy sediments. Among the four siboglinid lineages, only Vestimentifera has a representative with a sequenced genome, therefore a large piece of the puzzle in these deep-sea symbioses remains missing. In this study, we propose to provide high-quality genome assembly for both the host and symbiont genomes for an undescribed species of Sclerolinum collected from a cold seep in the South China Sea. Our preliminary analysis shows that the symbiont of this species of Sclerolinum is unique among all sequenced siboglinid symbiont genomes in that it is smaller in size, and has apparently lost the mobility and host invasion genomic components that are required for free-living. We will analyze the host/symbiont genomes with emphases on the material transfer between them, and test hypotheses of changes in the host genome that have allowed the symbiont to become host-restricted. We will compare the Sclerolinum holobiont with those of other siboglinids to unveil the diversity and evolutionary trajectories of symbiosis in deep-sea tubeworms. Moreover, we will compare the symbioses in tubeworms with those in other taxa such as mollusks to understand the independent acquisitions of symbiosis allowing them to thrive in deep-sea chemosynthesis-based ecosystems.